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Chuck Stone
03-04-2011, 8:11 AM
I figure that if anyone had a hint or shortcut, it might be here :D

I've been working on brass inlay (chemical milling) and having a bit
of trouble with registration in Corel. (could be any vector program for
all I know, but this is what I'm using)

To do the inlay, I need to mask both sides of the brass and the mask
must be cut in the same place on the reverse side, only mirrored.
So far I've been using grid lines to help with alignment, but for some
reason it isn't quite exact.

First I trace the artwork to get a vector, then cut the brass to size.
I enlarge the page and put in four grid lines just outside the artwork.
I lightly etch those onto a sheet of card stock (without the artwork)
This gives me a workspace on the table for placement.

I apply the mask to the brass leaving 1/4" border on each side and
engrave the front side with enough power to go through both sheets
of mask (front and back) The grid lines extend beyond the artwork
and also beyond the 1/4" border.

Then I mirror the image in Corel. I match up the grid lines that have
burned through the mask (viewed from the back side) and match
these up with the grid lines on the card stock, then run the file again.

So far it is coming out close, but not as exact as I would have thought.
It can be off by 40-50 thousandths .. too much for an inlay. OR it might
only be 0.010" .. and I'm not sure why.

Does Corel really mirror without changing placement? Or is there some
secret function going on that I'm not aware of? (and therefore not
compensating for?) I thought this would be a good method for getting
the front/back registration, but so far the results are hit-or-miss.

I also run the raster image in the wood just deep enough to fit the brass
and it is very satisfying to feel the inlay drop in like a puzzle piece.
If you look near the "P" you can see a slight double edge to some of
the lines. That is the back side or the mirror image showing through.

185098

Mike Null
03-04-2011, 9:00 AM
Chuck

I suspect that your image is not symmetrical therefore when you mirror it is off by the amount you have indicated. If this is a vector image it would be quite easy to redraw it so that it is symmetrical. Even if it is not a vector drawing making a new one wouldn't be that tough.

Dan Hintz
03-04-2011, 10:07 AM
I'm with Mike. As a test, place a line right in the middle of your graphic... in Corel, verify it's right in the middle of your grid lines, too. Engrave just that tic mark. Flip it over and re-engrave just the tic mark. Do they line up perfectly?

Now do the same, but instead of a tic mark in the middle of the graphic, do one on either side of the graphic, just touching it. Again, make sure the distance between each tic mark and the grid line is equal. Engrave tics. Flip. engrave tics again. Do they line up perfectly?

Chuck Stone
03-04-2011, 10:54 AM
Thanks for the replies.. this particular example *is* symmetrical, but it shouldn't matter.
What matters is that the grid lines behind the graphic are symmetrical. (within the image
area..)

The grid lines are evenly spaced 1/2" from the edges of the file, and 1/2" from the top
and bottom. In theory, no matter what else is in the file it should match up when the
image is mirrored, right? Or at least I thought so..

Mike Null
03-04-2011, 11:16 AM
Chuck

Check the xy position of the drawing before and after you mirror it.

Dan Hintz
03-04-2011, 11:23 AM
Chuck,

Run my test and I think you'll be surprised by the result...

Mike Null
03-04-2011, 11:32 AM
I now suspect that the positioning of the mask, which I understand you are doing by hand, is the problem.

I would not have traced this artwork--it is too inaccruate and too easy to draw your own vector art which would be more reliable.

Chuck Stone
03-04-2011, 11:54 AM
ok, I can do those things.. but it may not be hard for YOU guys!
(even after all these years, I still can't think in Corel..)

But what's throwing me is that I can't see *why* symmetry should be
an issue.. or why the graphic itself should matter.. traced or otherwise.
It could be a piece of on a purple background for all that matters
(at least in my head..) when I flip it, it should be a mirror image of the
fly stuff on the purple background..
and if the grid lines are matching up I'm not sure how the graphic wouldn't.

I'll try it, but probably not today.. I'm getting on the road to go pick up
an induction casting machine in Ontario. Got a lot of driving to do over the
next couple of days. (but should be able to start making some custom pen
parts soon ) :D

Real Mercier
03-04-2011, 1:20 PM
Chuck:

Have you viewed the graphic in wireframe mode? You could have a stray node skewing the center of your graphic and when you mirror it, it will skew the whole graphic. It doesn't take much. I would look at the wireframe, zoom in and check all the way around. Stray nodes happen a lot when you trace jpegs due to the anti-aliasing.

Dee Gallo
03-04-2011, 1:33 PM
Chuck, is there a chance that the tiny discrepancy has to do with the kerf of the cut? Are both the positive and the negative parts perfectly vertical at the edges? Is there a chance that the metal has shrunk a smidge from the original drawing due to cooling off?

Just throwing it out there, I'm probably way off, :) dee

Mike Null
03-04-2011, 1:41 PM
In this case the reason symmetry is important is that you are cutting both masks at the same time as I understand it. Then you are applying the second mask to the back side.

I may not understand your process but if the mask is a stretchable material that could be an issue or if you're going to cut both masks at the same time you need to cut one from the back side.

If you cut the mask one from the front the other from the back then I don't know why you'd need to mirror the image. Maybe I'm just confused.

Chuck Stone
03-04-2011, 2:14 PM
Hey guys...

I'm applying a sheet of mask material to both sides of the brass. I etch the lines THROUGH both
pieces that hang over the edge of the brass (so that I can see them when i flip it over) and
do the artwork. I can see that the grid lines worked OK, since they line up. Then I turn it over,
align the etched lines with the card stock again and etch the reversed artwork.

I'm running the file twice.. once for the front and once for the back. (reversed)

No stretching involved as far as I know (but would I be able to see it anyway?)
Dee.. I don't *think* it is the cut kerf, as sometimes I can get it to line up correctly
(in my limited experience.. all of about a half dozen tries so far..)

Real (Hi Real!) I didn't even know there *was* a wireframe mode.. but as for the
anti-aliasing, by the time i get it into Draw, the only thing I do with it is print or
mirror. Even if it is all screwed up somewhere, wouldn't it simply screw up in reverse?
(which would be OK .. what is important here is that the front and back match exactly
even if it is wrong..)
I'm using Photoshop for the raster image, I save it and import it onto Draw, then Trace.
The only thing I do after that is print it or mirror and then print it. So whatever Trace
is doing.. shouldn't change just because I mirror it, should it?

Dan .. did the test. tick marks line up OK. I wonder if I'm just having trouble seeing where
to line up the piece? From the front of the laser I'm basically viewing it at a 45 degree
angle. I can line up the vertical lines just fine (or so I was thinking) but the horizontal
is trickier. I have to move around to the side of the unit to see where to move it up or
down. (or have a second person to play monback )

Trying to match up lines on the flat cardstock and the brass shim (which is at a different
height) is giving me trouble, but I thought i had it right.

I'm soooo close. If I could just figure out placement/registration, I think I could
do some pretty intricate detail. Would be nice for box corners and such..

Mike Null
03-04-2011, 2:36 PM
Chuck

I was/am confused.

I apparently misread the following to believe you were cutting both mask applications at one time.
I apply the mask to the brass leaving 1/4" border on each side and
engrave the front side with enough power to go through both sheets
of mask (front and back) The grid lines extend beyond the artwork
and also beyond the 1/4" border.

I didn't realize you were talking about the mask and the carrier and not both masks.

Dan Hintz
03-04-2011, 3:07 PM
Dan .. did the test. tick marks line up OK.
The tic marks are no different than the image, so if they line up, then the image should. If the tic marks do, but the image doesn't, there's some odd step in your alignment procedure causing this (though I'm still clueless as to how your alignment procedure works). Pics would help here...

Mike Null
03-04-2011, 3:16 PM
If I am now understanding correctly, you are moving the work piece to engrave the back side. I believe that is your problem.

I don't understand your masking process. Can you elaborate. You say you engrave through both sheets of mask????

Chuck Stone
03-04-2011, 3:53 PM
I'm using an adhesive polyester sheet. I lightly score a piece of cardstock for
alignment. I put the polyester on both sides of the brass, with about 1/4 or 1/2"
overhang on all four sides. (depends on the piece) then i score through both
sheets at once so that i already have something on the back side to use as a
visual alignment tool. If I didn't go through the back sheet, then I'd have nothing
to see when i turned the piece over to do the other side.
When I flip it over, I use the lines that burned through the overhang to place
the piece and then cut the second side.

Grid lines are matching up, so it must be my placement, then. I'd use the rulers
on the laser, but you can never cut sheet metal by hand and get 90 degree
corners.. (at least not me..)

I do have to move the piece. As hard as it is to place the work piece this way,
it would be impossible for me to try to put an already engraved mask on the
brass and have it line up with the reverse side..http://www.sawmillcreek.org/images/misc/pencil.png


185130185131

Mike Null
03-04-2011, 4:22 PM
Do you have a metal shear? If so I can tell you how to align it if you want to give me a call.

If you don't have one I recommend you buy one. Every now and then they're on ebay.

Real Mercier
03-04-2011, 6:11 PM
Chuck:

Here is just a thought. Is the problem the thickness of the brass? In other words, you place a bottom layer of mask that lays flat, next you have the brass which is offset by 1/4", and then you lay another layer over that aligned with the fist mask layer. The second mask layer has the additional length of the brass thickness to follow. That would shorten the length of teh top layer.

Richard Rumancik
03-06-2011, 10:06 AM
. . . Does Corel really mirror without changing placement? Or is there some secret function going on that I'm not aware of? (and therefore not
compensating for?) I thought this would be a good method for getting
the front/back registration, but so far the results are hit-or-miss. . . .

Chuck, as far as I have seen, the mirror function works accurately. To prove it to yourself, draw an arbitrary shape (non-symmetrical). When you select the shape using the pick tool (arrow) there will be two boxes in the top left corner showing the Object(s) Position.

Record the x,y coordinates, then use the transformation tool to mirror the shape about its center left-to-right (about a vertical axis passing through it's center). You will see that the x,y coordinates do not change upon mirroring.

I think you have a pretty good handle on the theory. But the devil is in the details. When I get a chance I'll make a few suggestions as to how you might improve the process.

Mike, a metal shear might be a handy tool, but it will not help much in this case. To do what Chuck needs to do, you need to work from centerlines, not edges. You cannot use rulers and part edges as datums in this case, as when you flip you will be contacting the
opposite edge. Trying to make the brass outline precise would be very difficult and not really practicable.

The shape of the brass sheet and the shape of the polyester lamination are completely arbitrary and need not be cut accurately at all.

Chuck, you are probably getting a maximum registration error of less than .010" but you really need to work towards getting .002" or better. As well, you want a process that offers repeatibility. Getting .002" one time and .012" the next is not suitable for production as there is too much work involved only to find the part is scrap.

Achieving .002" or better will be challenging using a visual method of alignment, but you might be able to improve your process a bit. I'll try to write something up.

Dee Gallo
03-06-2011, 10:58 AM
Chuck, you might want to use a different registration process. A more accurate method is to cut registration holes and place pegs where they go. Then, when you slide the piece onto the pegs, there is no question whether you have visually lined up or not.

cheers, dee

Rodne Gold
03-06-2011, 11:02 AM
If you running thin brass as an inlay , you need only mask one side with the graphic ,.you could use a solid resist on the back or if real thin , no resist on the back , you would etch the plate upside down and spray the face with hot ferric with 5% citric acid (Google Edinburgh etch) till it etches right thru
You would get some undercutting but that would work in your favour for the inlaying. You can use those small micro irrigation things you use in the garden for the spray nozzles , you need to get an acid resistsnt pump from a pump supplier and use a sump (like a polyprop container) use 300w aquarium heaters to get to about 40c. You can etch 1mm (1/25th) brass right thru in about 5 mins if your ferric is the right baum no and temp.

Also Look at this
http://www.prototrains.com/etch1/etch1.html
http://www.prototrains.com/etch2/etch2.html

Mike Null
03-06-2011, 11:41 AM
Richard

I disagree as a shear will enable him to make square cuts on pieces sized for the job and therefore allow him to use the rulers for more accurate positioning. No re-positioning is likely to be perfect but at least this will give him the best available for this job. It surely will not be off by .010".

Chuck Stone
03-06-2011, 9:59 PM
Thanks for the ideas, everyone.. there's a lot to digest. I'm not
in the position to go out and buy equipment to do this, and I think
a shear might be overkill for pieces this size. (but it would go great
in my shop..) but I had better luck wearing a magnifying headset.
(I'm half blind, so I'll take all the help I can get)
The rulers are not much help for small work, and harder for me to see
so the grid lines and cardstock work OK. White stock, black lines are
easier than black ruler and dark gray, thick lines. I use the rulers for
gross placement, then the cardstock for finer placement.

I'm not sure how the pin arrangement would work for me.. if it isn't
movable then I might be limited to a certain size/placement? and if
it is movable, then I'm back to the same issue of placing the pin jig
instead of the brass stock. (or maybe I don't understand it yet.)

I'll keep working on it and see how I can incorporate some of these
ideas.

Thanks for all the help!

Mike Null
03-07-2011, 3:42 AM
Chuck

The rulers would do the same thing as the pins--fix the position of the work so you are working with a constant. But the work piece has to be constant as well.

Richard Rumancik
03-07-2011, 12:22 PM
Dee, you are on the right track with the holes and pins (pegs) idea.

I'll try to explain with a CNC router analogy.

If a person needed two machined patterns accurately aligned on two sides of a workpiece, and this was to be done on a CNC router, this is what you could do:

1. Attach a spoilboard rigidly to the table.
2. Place the brass on the spoilboard and clamp it to the table.
3. Drill two alignment holes through the brass into the spoilboard. The alignment holes would be placed exactly on the shape centerline, and located above and below the shape.
4. Machine the pattern from the front side.
5. Remove the clamps and brass plate, and install 2 pins in the spoilboard holes.
6. Flip the sheet over, and place it on the pins.
7. Machine the rear pattern (which could be the mirror-version of the front pattern.)

This will ensure front and rear patterns align.

Note that the shape and accuracy of the workpiece is irrelevant. Symmetry of the shape is irrelevant.

Now the trick is getting this to work with the laser. The laser cannot cut through brass.

So what Chuck did is attach polyester to the brass. He can cut through polyester. He needs the polyester anyway as an etch resist.

Rather than use two center holes Chuck drew a broken "rectangle" around the shape, (having the same center as the shape.) This works similar to a centerline, although my preference would be to use two holes for registration rather than the lines.

His "spoilboard" is the cardstock. Rather than pins, he is using a visual alignment method (which of course is not as good, but is much easier.)

I think that using the visual method, you can get within .005" error if done carefully. Maybe this is good enough for what Chuck needs.

But to get better than .005" error you need pins. I have used pins, and it works very well. But of course you need a fixture. However, an acrylic fixture made on the laser can work quite well. The very best precision requires a metal fixture.